This invention relates generally to process control apparatus and more particularly to a first level zone condition control apparatus for controlling the operation of heating, ventilating, humidifying and air conditioning (HVAC) equipment to effect environmental conditioning.
Computerized control systems are in wide use for controlling a variety of processes, petrochemical, power generation and steel making being exemplary. Typically, each such process comprises a plurality of process subsystems or zones which may have control requirements peculiar thereto and which may be controlled by an apparatus dedicated to the proper functioning of the particular zone. Such controllers are usually coupled to a more sophisticated, master controller disposed at a second or higher hierarchical level within the overall control system and coupled to the first level controllers by a communication bus. For processes having a relatively small number of parameters to be sensed and controlled, it is desirable to employ a zone controller which is constructed and arranged to define a less sophisticated and therefore less expensive apparatus which is carefully matched to the parameters of the zone being controlled.
Another type of process which may require zone control or which may be adapted to control by zones includes environmental processes related to heating, ventilating, humidifying and air conditioning. Sophisticated and relatively expensive hierarchical systems are known and in common use for controlling the HVAC systems in locations such as large office buildings, universities, industrial complexes and the like. Such systems will usually include a master control computer coupled to a number of subcontrollers which are distributed throughout a building and are capable of more limited computerized function. These subcontrollers, in turn are typically coupled to a number of individual modules and load devices for controlling the HVAC equipment. While these systems provide highly acceptable performance in those installations, their complexity and resultant cost makes them undesirable for use in smaller HVAC processes such as might be found in supermarkets, smaller warehouses, office buildings and the like. In constructions of this latter type, the HVAC equipment will typically include only a single set of interrelated ducts, termed an air handling unit, coupled to a zone or space, the temperature and/or humidity environment of which is to be controlled. While these spaces are most frequently intended for human occupancy, such spaces may be devoted to the storage of food or other goods requiring a closely-controlled ambient.
The ducts comprising the air handling unit are constructed and arranged for drawing outdoor ambient air into the space, for exhausting air from the space to tne outdoor ambient and for controllably mixing intake and return air. Such air handling units are equipped with adjustable dampers for controlling the flow of air and include heat dissipating or absorbing coils formed of tubing placed in the air flow path within the duct. These coils may be arranged in two sets, one each for flowing heated or chilled water or refrigerant therethrough, thereby permitting the duct air temperature to be raised or lowered. Valves are provided for controlling water flow. These dampers and valves may be controllably positioned by load devices such as motorized rotary actuators coupled thereto.
In the alternative, a water-type heater coil may be replaced by a group of electrically-powered heater strips which may be energized in one or more stages for air heating. These heater strips may be energized by a load device such as a sequencer in response to command signals received by it. Control of the load devices, the exemplary actuators and sequencers, may be in discrete stages or in a continuum.
In systems of this type, energy savings may be realized by incorporating economizing functions within the control scheme. For outdoor air temperature and humidity which fall within a predetermined band of temperature and humidity values, the cooling effect inherent in the outdoor air may be utilized for appropriate conditioning of the space while yet avoiding the expenditure of supplemental energy for this function.
One type of microcomputer-based zone controller incorporates a small number of resistive elements, the output signals of which are used by the controller for selecting the desired setpoints of certain process parameters or for sensing temperature and relative humidity values. Such controllers are arranged around a centralized intelligence concept; that is, the data management and computational algorithms are embodied exclusively in the controller microcomputer or microprocessor and associated memory functions. The load devices to be connected to and operated by the controller include no provision for device programming, memory function or communication with the controller. Such a controller incorporates one or more multiplexers for sequentially directing sensed and setpoint parameters to a microcomputer for processing. Signals so processed are inverted and used to selectively energize one or more of a plurality of controller-mounted electromagnetic relays for actuating staged heating, cooling or a combination thereof. Other signals as, for example, from a heat/cool changeover switch are inverted, directed to a comparator network and used to positionably control a rotatable actuator for powering outdoor air dampers. A potentiometer is used to select that actuator position which provides the minimum air flow required for ventilation. With such a controller, each heating and/or cooling stage would be coupled to a predetermined set of relay output contacts by a pair of wires, both the contacts and the wire pair associated therewith being dedicated solely to the task of controlling the particular stage coupled thereto. Economizer control by a method known as differential enthalpy may be accomplished only by connection of a separate enthalpy control module to the controller. An example of such a zone controller is shown and described in U.S. Pat. No. 4,347,712.
While such zone controllers have hitherto provided a satisfactory means for controlling HVAC equipment, they tend to be characterized by certain disadvantages. In particular, each controller relay contact is required to have a pair of dedicated wires coupled between it and the associated heating or cooling stage. The analog output terminals for controlling the economizer motor are likewise required to have a pair of dedicated conductors coupled thereto. Since the distances between the controller and the economizer motor or heating and cooling stages may be substantial, the cost of installing this wiring, eight or more conductors in all, may be quite significant. Additionally, the controller microcomputer, having a predetermined number of input/output ports, may be bound by the number of such ports to a maximum number and type of load devices connected to the controller. Therefore, if a process control application requires an output configuration other than that which may be available from the predetermined number of relay output contacts and analog output terminals, it will be necessary to modify the controller and its self-contained hardware in order to accommodate the controller to such an application. It is difficult or impossible to adapt a controller of this type to a system wherein the combined number of heating and cooling stages exceeds the relatively limited number of electromagnetic output relays embodied in the controller. Another disadvantage of such a controller relates to the fact that HVAC air handling units tend to have varying numbers of heating, humidifying and/or cooling stages required from application to application. Notwithstanding, it may be highly desirable from comfort and energy conservation standpoints to cause the progressive energization or de-energization of the heating and cooling stages to occur in evenly-spaced increments across the width, in temperature degrees, of the heating or cooling proportional bands, irrespective of the width of these bands or of the number of heating or cooling stages. Known zone controllers are somewhat inflexible and therefore not easily adapted to such operating environments.
Yet a further disadvantage of a controller of this type which uses a system of centralized intelligence is that no means are included whereby the controller may selectively poll or otherwise communicate with the load devices coupled thereto. The controller is therefore unable to identify, by digitally coded signals, the precise type of load device coupled thereto nor to interrogate and receive signals from the load devices indicative of their respective positions or status.
A zone controller which utilizes a microcomputer and is adapted to control microcomputer-based load devices such as actuators and sequencers to form a system having distributed intelligence, which is capable of communicating with any one or all of a plurality of such load devices by a single, two-wire communication bus coupled therebetween, which is adapted to communicate with such load devices by a pair of microcomputer digital signal input/output (I/O) ports and which is adapted to communicate with a central processing unit at a higher hierarchical level would be a significant advance over the prior art.